1. Field of the Invention
The present invention relates to switching control executed by a transmission apparatus such as a BLSR (Bi-directional Line Switched Ring) multiplexing apparatus having a function to switch a line in the event of a transmission failure.
2. Description of the Related Art
In a network including a BLSR multiplexing apparatus, a transmission line is designed into a redundant configuration comprising a working line and a protection line. If a failure occurs on the working line, the transmission line is switched to the protection line for transmissions in a direction opposite to the working line so as to improve reliability of the network. The working line is a line that is used when no failure occurs. The protection line is a line to which the transmission is switched from the working line in the event of a failure on the working line. This BLSR multiplexing apparatus is used as an apparatus for transmitting data such as voices and pictures in a synchronous network such as an SDH (Synchronous Digital Hierarchy) network. In the case of a BLSR multiplexing apparatus, a band between addition of a signal to a transmission line and dropping of the signal from the transmission line accommodating a terminal on the reception side is allocated to the transmission line as a channel. Since the channel is never allocated to the transmission line once the data has been dropped from the line, however, the transmission capacity of 1 optical fiber is, on the average, 1.5 times that of a transmission apparatus used so far with the channel occupying a transmission line of 1 ring cycle. The addition of a signal to a transmission line cited above is an operation to transmit the data output by a terminal on the transmission side through the line. On the other hand, the dropping of a signal is an operation to transmit the added signal to a transmission line connected to a terminal on the reception side or another ring.
In particular, in a 4-fiber BLSR multiplexing apparatus, each transmission line is a shared line used as both a working line and a protection line. A 4-fiber BLSR multiplexing apparatus is used in conjunction with 4 transmission lines, namely, 2 working lines and 2 protection lines. Each of the 4 transmission lines is a physically separated fiber. In comparison with a 2-fiber BLSR multiplexing apparatus, a 4-fiber BLSR multiplexing apparatus has a span-switch function in addition to a ring switch for switching a transmission from a working line to a protection line in the event of a failure on the working line. It is thus quite within the bounds of possibility that the network is rescued from a line failure. Accordingly, the demand for a 4-fiber BLSR multiplexing apparatus is rising. In addition to the conventional multiplexing apparatus, for the BLSR multiplexing apparatus, there have been discussed a network configuration for transmission over a long distance through a sea-bottom cable and a switching technique referred to as a submarine BLSR. By carrying out a switching operation according to an adopted technique in the event of a failure on a transmission line, suspension of a signal transmission can be avoided.
In order to execute switching control in the event of a failure in a conventional BLSR multiplexing apparatus which is also referred to as an NE (Network Element), every BLSR multiplexing apparatus is provided with a ring topology table and a squelch table showing a range of cross-connected working lines for each channel such as an STS1 or VC3 channel. A ring topology table is a table showing a connection order of NEs connected to form a ring in a direction toward an east side starting with the node ID of this station. The direction toward the east side is defined typically as a counterclockwise direction along the ring. A node ID is an ID assigned to each NE uniquely in the network. By referring to a ring topology table, each NE is capable of knowing the node ID of an adjacent NE.
A squelch table is a table used in a BLSR multiplexing apparatus for forming a judgment as to whether or not to insert an AIS signal into a circuit that cannot be rescued from failures when the failures occur on a plurality of transmission lines in order to avoid incorrect connection. Information stored in a squelch table includes the ID of a signal-adding NE and the ID of an NE dropping the signal. In this squelch table, an add channel and a drop channel are set individually only for each working line. An add channel is the channel of one of 2 working lines, which is used for transmitting added data. A drop channel is the channel of a line dropping a signal. The node ID of a signal-adding NE is referred to as a source-node ID. On the other hand, the node ID of a signal-dropping NE is referred to as a destination-node ID.
The user enters cross-connect information for each channel to an NE on the channel. The cross-connect information includes Add, Drop or Through (or Relay) and the direction of the signal such as East or West. Each NE stores a source-node ID or a destination-node ID in a SONET or SDH overhead byte to be used in creation of a squelch table, and transmits as well as receives node IDS in accordance with the following rule. A transmission table and a reception table are provided in a squelch table for an add channel and a drop channel respectively. The transmission table is a table that is used for storing a node ID when the node ID is transmitted. On the other hand, the reception table is a table that is used for storing a node ID when the node ID is received.
A signal-adding NE sets the node ID of its own as a source-node ID in the transmission table for the channel used for transmitting the added data. The signal-adding NE stores a received destination-node ID in a destination entry of the transmission table and a destination entry of the reception table before bouncing back the destination-node ID to the adjacent NE. The signal-adding NE stores a source-node ID bounced back by a signal-dropping NE in the reception table. A data-passing-through NE passes on a received node ID to an adjacent NE as it is. A signal-dropping NE sets a destination-node ID in a transmission table for a signal-dropping channel. The signal-dropping NE stores a received source-node ID in a source entry of the transmission table and a source entry of the reception table before bouncing back the source-node ID to the adjacent NE. The signal-dropping NE stores a source-node ID bounced back by a signal-dropping NE in the reception table. Each NE determines that the creation of a squelch table has been completed when the transmission table is found to match the reception table. If the transmission table does not match the reception table, on the other hand, incorrect setting is determined to exist. In this way, incorrect setting of each channel can be avoided. In addition, an AIS signal is inserted into a circuit that cannot be rescued from failures when the failures occur on a plurality of transmission lines, making it impossible to carry out communications between a signal-adding NE and a signal-dropping NE, which are set in the squelch table.
A submarine BLSR multiplexing apparatus supports a drop-and-continue connection as an inter-ring connection. A drop-and-continue connection is a connection by which a primary node drops a transmitted signal to an adjacent ring network and, at the same time, relays the signal to an adjacent NE. Two drop-and-continue options are available. One of the options is selected for passing through a secondary circuit of a redundant system to a working line and the other option is selected for passing through a secondary circuit of a redundant system to a protection line. The former is referred to as a DCW (Drop and Continue on Working) connection while the latter is referred to as a DCP (Drop and Continue on Protection) connection. In a DCP connection, the primary node drops a signal from transmission to an adjacent ring network but continues the signal to a protection line before a secondary node drops the signal from transmission to the adjacent ring network. In a DCP connection, a primary node is a node that is connected to an adjacent ring network and plays roles to drop a working signal from transmission to the adjacent ring network and continue the signal to a protection line. On the other hand, a secondary node in a DCP connection is a node that is connected to an adjacent ring network and plays a role to drop a working signal from transmission to the adjacent ring network through a protection line.
In a DCW connection, on the contrary, the primary node drops a signal from transmission to an adjacent ring network but continues the signal to a working line before a secondary node drops the signal from transmission to an adjacent ring network. In a DCW connection, a primary node is a node that is connected to an adjacent ring network and plays roles to drop a working signal from transmission to the adjacent ring network and continue the signal to the working line. On the other hand, a secondary node in a DCP connection is a node that is connected to an adjacent ring network and plays a role to drop a working signal from transmission to the adjacent ring network through the working line.
As another implementation of connection between ring networks, there is provided a DTP (Dual Terminal Transmit on Protection) connection in which a terminal node accommodating a terminal adds a signal to a working line as well as a protection line, a primary node drops a signal from a working line and a secondary node drops a signal from the protection line. As further implementation of connection between ring networks, there is also provided a DTW (Dual Terminal Transmit on Working) connection in which a terminal node accommodating a terminal adds a signal to a working line and a secondary node drops a signal from the working line.
However, the conventional BLSR multiplexing apparatus has the following problems:    1: Since a squelch table is created as a collection of only source-node IDs and destination-node IDS, it is not possible to express a variety of implementations of connection such as the DCW connection and the DCP connection. Thus, for example, a connection of switching in the event of a failure for the DCW connection is the same as that for the DCP connection, making it impossible to carry out a switching operation according to the implementation of connection.    2: A squelch table is created as a collection of only node IDS of signal-adding NEs connected to working lines and signal-dropping NEs also connected to working lines without collecting node IDs of secondary nodes connected to protection lines. Since squelching control is executed in accordance with such a squelch table, an unnecessary squelching operation is executed for a DCP connection or a DTP connection even if the network can be rescued from a transmission-line failure.    3: Since a connection is checked incorrectly due to the fact that only node IDs are collected, it is not possible to easily check whether or not a node ID of a node between destination and source nodes is correctly set. It is thus also impossible to carry out a complex checking operation according to the implementation of connection such as the DCP or DCW connection.